Bitterness in the bible is similar to poison and causes us to be defiled before God. (Hebrews 12:15) Bitterness is an underlying problem that doesn't always manifest on the outside, but dwells in that person's system.
Paul is an amazing example to someone who did not hold any roots of bitterness when he was betrayed and harmed. Do you remember the story of Paul, Barnabas and John (Mark)?
Some time later Paul said to Barnabas, “Let us go back and visit the believers in all the towns where we preached the word of the Lord and see how they are doing.” Barnabas wanted to take John, also called Mark,with them, but Paul did not think it wise to take him, because he had deserted them in Pamphylia and had not continued with them in the work.
Here we read how John (Mark) deserted Paul and Barnabas in an earlier occasion. He cowered away and didn't help finish the work in which he was sent out to do. Honestly, if someone deserted you in the heat of a dangerous and life threatening situation, wouldn't you feel a bit bothered? I'm pretty sure we all would. But... here is the beautiful thing.
My fellow prisoner Aristarchus sends you his greetings, as does Mark, the cousin of Barnabas. (You have received instructions about him; if he comes to you, welcome him.)
Paul tells the other churches to welcome Mark if he were to come to them. Paul could have held on to resentment and commanded that the church close it's doors to Mark and show him no hospitality, but that wasn't the case.
I want Paul and Mark's situation to be an example to me. When someone offends me, I want to be able to forgive them and not to allow roots of bitterness to take root in my heart. Will you do the same? Don't let roots of bitterness to take root in your heart, grow and prosper, producing a harvest of sin.